Since the discovery of neural stem cells, a connection between these cells and the origin of brain tumors has been theorized. The recent discovery by us and others of brain tumor stem cells and their close phenotypic relationship to neural stem cells has prompted us to explore proliferative mechanisms that are in common between neural stem cells and brain tumors. We have previously found that maternal embryonic leucine zipper kinase (MELK), a serine-threonine kinase, is a marker for and regulates the proliferation of self- renewing neural progenitors (neural stem cells) in developing mouse brain. We have also found that MELK expression correlates with brain tumor grade and survival and that it is a "hub" gene;one whose expression predicts the expression of numerous other cell cycle genes. We hypothesize that MELK is an important regulator of brain tumor proliferation, mediates the self-renewal of brain tumor stem cells and serves as a marker for brain tumor stem cells. However, further studies are needed to determine if MELK and its putative molecular pathwaywill serve as targets for brain tumor therapy. This proposal will address these issues. We will first perform experiments to determine whether MELK mediates the proliferation of brain tumor cell lines, focusing on medulloblastoma and glioblastoma multiforme (GBM). These tumors were chosen because they are common and there is strong preliminary evidence supporting the hypothesis that MELK regulates their function. Also, since the presence of brain tumor stem cells in medulloblastoma has been questioned, it will be important to determine whether MELK functions in a different capacity in these tumors compared to GBM. We will establish the mechanisms underlying MELK effects by determining MELK's role in regulating the cell cycle, the genes that mediate its function, and whether this function is mediated by its kinase domain or the domain critical for a function in RNA processing. We will also determine whether MELK function is critical for brain tumor cell line proliferation in the in vivo environment of the brain. To determine whether MELK is expressed by cancer stem cells, we will first'test whether it is expressed by sphere-forming multipotent progenitors derived from tumors and then whether these progenitors are capable of tumor initiation. We will then determine the role of MELK in brain tumor stem cells in vitro and in vivo. Through these studies, we will determine whether MELK or its functional pathway is an appropriate target for novel therapies of brain tumors. Lay summary: A study of stem cell genes will lead to promising new treatments for brain tumors. This application focuses on a gene, MELK, that is a critical regulation of the proliferation (division) of normal neural stem cells and is expressed by brain tumors. We will determine the function of MELK in brain tumor cells (glioma and medulloblastoma) and whether it is important for the proliferation of tumor-causing "cancer stem cells". If MELK is important for these functions then it becomes a target for potential treatments.